complete mastery; at one's beck and call
Idiomatic expression meaning to have something completely under one's control or at one's disposal, like a tool in one's own medicine cabinet. Often used in the pattern 〜を自家薬籠中の物とする.
He has complete mastery of that technique.
That company president has talented people at his beck and call.
Literally 'something in one's own medicine cabinet'. The phrase originates from the idea that a doctor keeps medicines in their own cabinet, ready to use at any time, and by extension refers to having something completely under one's control.